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The development of electronic materials for storing electrical energy is a thriving research field, where the materials used in batteries, supercapacitors, and dielectric capacitors have attracted extensive interest in last decades. The dielectric capacitors showing unique characteristics such as high power density and large charge/discharge rate have been actively studied, where the antiferroelectrics demonstrate great potentials for dielectric energy storage applications by storing and releasing energy upon a reversible electric-field induced antiferroelectric- ferroelectric phase transition. Recently, lead-free antiferroelectric AgNbO3 has emerged as a promising candidate to substitute conventional lead-based antiferroelectrics (such as PbZrO3) in energy storage applications. The phase transition dynamics of AgNbO3 is driven by a complex sequence of oxygen octahedron tilting orders in addition to cation displacement, which can be effectively engineered by a doping strategy. In this article, we present a succinct overview of the phase transition mechanisms in AgNbO3-based ceramics and describe how the phase transition characteristics are affected by the dopants. By exploring the composition related average structure and local structural evolutions, we provide a view toward the goal of establishing a link between the phase transition and physical properties tailored for dielectric energy storage applications. The development of electronic materials for storing electrical energy is a thriving research field, where the materials used in batteries, supercapacitors, and dielectric capacitors have attracted extensive interest in last decades. The dielectric capacitors showing unique characteristics such as high power density and large charge/discharge rate have been actively studied, where the antiferroelectrics demonstrate great potentials for dielectric energy storage applications by storing and releasing energy upon a reversible electric-field induced antiferroelectric- ferroelectric phase transition. Recently, lead-free antiferroelectric AgNbO3 has emerged as a promising candidate to substitute conventional lead-based antiferroelectrics (such as PbZrO3) in energy storage applications. The phase transition dynamics of AgNbO3 is driven by a complex sequence of oxygen octahedron tilting orders in addition to cation displacement, which can be effectively engineered by a doping strategy. In this article, we present a succinct overview of the phase transition mechanisms in AgNbO3-based ceramics and describe how the phase transition characteristics are affected by the dopants. By exploring the composition related average structure and local structural evolutions, we provide a view toward the goal of establishing a link between the phase transition and physical properties tailored for dielectric energy storage applications. [ABSTRACT FROM AUTHOR]

The influence of stress on the small‐signal dielectric permittivity and piezoelectric coefficient of polycrystalline lead‐free perovskite 0.92(Na1/2K1/2)NbO3–(0.08 − x)Bi1/2Li1/2TiO3–xBaZrO3 (x = 0, 0.02, 0.04, 0.06, and 0.07) was characterized under different constant uniaxial stress up to −200 MPa within a temperature range of −150 to 450°C, revealing stress‐induced suppression of the electromechanical response as well as shifts in the phase boundaries. For all compositions, the interferroelectric and ferroelectric–paraelectric phase transitions were shifted to higher temperatures under the uniaxial compressive stress. Interestingly, the sensitivity to the applied stress was found to increase with increasing BZ/BLT ratio in the system. The origin of a different extent of stress‐sensitivity with BZ/BLT ratio is suggested to be related to the change in the crystal structure. Additionally, at temperatures below −50°C, the relative permittivity showed a significant increase under applied compressive stress. [ABSTRACT FROM AUTHOR]

Similar to the canonical antiferroelectric (AFE) compound PbZrO3 in Pb(Zr,Ti)O3 solid‐solutions, the presence of double hysteresis loops and that of electric field–induced phase transitions are important characteristics of NaNbO3 AFE materials; yet the phase transition behavior in the latter system is typically irreversible with the related mechanisms not fully understood. Here, we explore the phase transition mechanism of ferroelectric and AFE phases in NaNbO3 based on measurements of single crystals with different directions in conjunction with density‐functional theory (DFT) calculations. The tilting and distortion behaviors of the [NbO6] octahedra are explained by DFT, and the ion displacement in the lattice is traced. The tilting and distortion behaviors of the [NbO6] octahedra with different orientations are compared. We confirm that the tilt and distortion of the [NbO6] octahedra along the [1 1 1] direction is the main reason to improve the stability of AFE phase. This conclusion is verified by the experimental characterizations of large‐size NaNbO3 single crystals successfully obtained in this study. [ABSTRACT FROM AUTHOR]

The past two decades have seen a great enhancement of piezoelectric coefficients (d33) to higher than 570 pC/N in (K, Na)NbO3 (KNN) piezoelectrics, but one notoriously unresolved issue is their severe temperature instability, obstructing them toward practical applications. The present work demonstrates a facile approach to overcome this problem by introducing a layered distribution of key dopants (Li and Sb) in a monolithic ceramic, featuring stepwise varied polymorphic phase transition (PPT) temperatures along the thickness direction. The dopant‐graded ceramic exhibits an outstanding d33 of 508 pC/N and a very large piezoelectric strain (Suni, of 0.18%). More importantly, an excellent temperature stability (d33 variation within 13% over the temperature range of 25–150 °C) is achieved, which is superior to that of most state‐of‐art KNN counterparts. These are attributed to the construction of spatially diffused PPT in combination with enhanced polarization, permittivity, and piezoresponse through interfacial effect, including the Maxwell–Wagner effect and intergranular stress by gradient doping. The results offer an alternative strategy for designing high‐performance piezoelectric materials with desirable temperature reliability. [ABSTRACT FROM AUTHOR]

Here, we introduce phase change mechanisms in lead-free piezoceramics as a strategy to utilize attendant volume change for harvesting large electrostrain. In the newly developed (K,Na)NbO3 solid-solution at the polymorphic phase boundary we combine atomic mapping of the local polar vector with in situ synchrotron X-ray diffraction and density functional theory to uncover the phase change and interpret its underlying nature. We demonstrate that an electric field-induced phase transition between orthorhombic and tetragonal phases triggers a dramatic volume change and contributes to a huge effective piezoelectric coefficient of 1250 pm V−1 along specific crystallographic directions. The existence of the phase transition is validated by a significant volume change evidenced by the simultaneous recording of macroscopic longitudinal and transverse strain. The principle of using phase transition to promote electrostrain provides broader design flexibility in the development of high-performance piezoelectric materials and opens the door for the discovery of high-performance future functional oxides. Functional oxides with coexisting states of comparable energy typically exhibit extraordinary responses to external stimuli. Here, the authors demonstrate that coexisting phase structures provide large electric field-triggered volume change. [ABSTRACT FROM AUTHOR]

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead‐free Li‐modified Na0.5K0.5NbO3 ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic‐tetragonal (M‐T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress‐ and temperature‐dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress‐induced M‐T phase transition in Li‐modified Na0.5K0.5NbO3. A stress‐temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M‐T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure. [ABSTRACT FROM AUTHOR]

Electrical poling is indispensable for endowing isotropic ferroelectric polycrystals with a net macroscopic polarization and hence piezoelectricity. However, little attention has been paid to the optimization of poling conditions in (K, Na)NbO3-based ceramics with a polymorphic phase transition. This study investigated the electrical properties of CaZrO3-modified (K, Na, Li)(Nb, Ta)O3 lead-free piezoceramics as a function of the poling temperature. Peak piezoelectric coefficient d33 of 352 ± 7 pC/N and planar electromechanical coupling factor kp of 0.47 were obtained at the optimized poling temperature of 120 °C, which crosses the polymorphic phase transition regime. In-depth analysis of the asymmetric polarization hysteresis loops and bipolar strain curves uncovered striking analogy between electrical poling and unipolar cycling in the current system, which is attributed to a competition between domain reorientation and space charge accumulation. [ABSTRACT FROM AUTHOR]

We report on temperature dependence of local electromechanical properties of lead-free (K,Na)NbO3-based ceramics that macroscopically manifests a large temperature-insensitive strain. Piezoresponse force microscopy reveals the particular role of the orthorhombic-tetragonal phase transition, where a reconstruction of the domain structure occurs and local piezoelectric response shows a peak value. A good quantitative agreement between temperature dependences of the local and previously reported macroscopic small-signal piezoelectric coefficients is observed. An influence of the polymorphic phase transition on polarization switching kinetics was revealed. [ABSTRACT FROM AUTHOR]

The orientation modulation of ferroelectric materials is a suitable method to optimize material performance. Textured Bi1‐xSmxFeO3 thin films (near the rhombohedral‐orthorhombic (R‐O) phase boundary, that is, x = 0, 0.1, 0.12, 0.14, and 0.16) were fabricated using the sol‐gel process by introducing a LaNiO3 (LNO) seed layer. Structural and ferroelectric characterizations were used to investigate the effect of texturing on the Sm doping‐induced R‐O phase transition of the BiFeO3 thin films. It was found that a phase transition occurred from the rhombohedral to the orthorhombic structure with increasing Sm content in the nontextured polycrystalline films, resulting in an R‐O phase boundary at x = 0.12. In contrast, the R‐O phase boundary in the textured films was more diffuse, indicating a two‐phase coexistence in a boarder range of Sm doping levels (x = 0.12‐0.16). This discrepancy was attributed to the complexity of the stress status in thin films. The dielectric and electrical properties of the nontextured and textured samples were investigated. The current study shows that the phase boundary in ferroelectric thin films can be altered by diverse means, thus providing insights into potential applications. [ABSTRACT FROM AUTHOR]

The effect of uniaxial compressive stress on the crystal structure of a 6 mol. % Li-doped (K,Na)NbO3 (LKNN6a) ceramic was investigated using in situ synchrotron X-ray diffraction, revealing the stress-induced relative change in monoclinic Pm and tetragonal P4mm phases. As such, stress-induced phase transformations, in addition to the lattice deformation and domain switching, are the contributing factors for the observed macroscopic mechanical behavior of LKNN6a. The in situ stress-dependent diffraction data also demonstrates a method to mechanically modulate the polymorphic phase transition temperature (TPPT) to a higher temperature, as observed by the temperature-dependent permittivity measurements under a constant bias stress. The external uniaxial compressive stress increases the stability of the lower symmetry monoclinic phase, shifting TPPT to a higher temperature by 60 °C for the maximum uniaxial compressive stress of 300 MPa in the studied composition. Importantly, the stress-induced stabilization of the room-temperature ferroelectric phase can be useful to optimize the phase transition region, as well as increase the temperature stability of lead-free KNN. [ABSTRACT FROM AUTHOR]

(K,Na)NbO3 (KNN) is a promising lead‐free ferroelectric/piezoelectric system, to which incorporating BaZrO3 can greatly enhance its piezoelectricity, but the mechanism is not clear. This work was conducted to investigate the phase transition in the BaZrO3‐modifed KNN system and its contribution to piezoelectricity enhancement, using thin films with a fixed orientation and high compositional homogeneity fabricated by a sol‐gel method. Two ferroelectric‐to‐ferroelectric phase transitions are revealed, which correspond to monoclinic MC‐ MA phase transition at higher temperature and rhombohedral‐monoclinic MC phase transition at lower temperature. It is difficult to distinguish these phases in KNN‐based bulk materials, but their differences are clear when conducting high‐resolution X‐ray reciprocal space mapping (RSM) on the present thin films. Piezoresponse force microscopy experiments also revealed an interesting finding that local piezoelectricity of monoclinic phases was higher than that of rhombohedral ones in KNN‐based thin films. This work could shed insights on the fundamental understandings for the effect of the chemical doping, and offer guidance for property optimization in the KNN‐based lead‐free piezoelectrics. Phase transition and piezoelectricity were modified in (K,Na)NbO3 based lead‐free piezoelectric thin films, where the R to monoclinic MC phase transition temperature was elevated with increasing BaZrO3 doping levels and the local piezoelectricity reached the maximum value at the phase boundary. [ABSTRACT FROM AUTHOR]

Monoclinic phases, as intermediate phases at the morphotropic phase boundary, have been considered as the origin of high piezoelectricity in various piezoelectric systems. However, those monoclinic phases have seldom been experimentally observed at the phase boundary in lead-free (K,Na)NbO3 (KNN)-based materials. This work shows that KNN epitaxial thin films demonstrate a monoclinic MC phase stabilized by elastic strain at room temperature, which is identified by both synchrotron X-ray reciprocal space mapping and piezoresponse force microscopy. It is revealed that the piezoelectricity is greatly enhanced at 170 °C due to the monoclinic MC-MA phase boundary. The discovery of monoclinic phases in this work may open an avenue for developing a new thermotropic phase boundary in KNN-based materials. The results also provide a guide for the exploration of high piezoelectricity driven by monoclinic phases in lead-free piezoelectric materials. [ABSTRACT FROM AUTHOR]

The temperature dependence of piezoelectric properties (direct piezoelectric coefficient d33, converse piezoelectric coefficient d33( E = 0), strain S and electromechanical coupling coefficient kp) for two niobate-based lead-free piezoceramics have been contrasted. 0.92(Na0.5K0.5)NbO3-0.02(Bi1/2Li1/2)TiO3-0.06BaZrO3 (6 BZ/2 BLT/92 NKN) has a morphotropic phase boundary ( MPB) between rhombohedral and tetragonal at room temperature and 0.92(Na0.5K0.5)NbO3-0.03(Bi1/2Li1/2)TiO3-0.05BaZrO3 (5 BZ/3 BLT/92 NKN) features an MPB engineered to be located below room temperature. At 30°C, d33, d33( E = 0), S (at 2 kV/mm), and kp are 252 pC/N, 230 pm/V, 0.069%, 0.51 for 5 BZ/3 BLT/92 NKN; and 348 pC/N, 380 pm/V, 0.106%, 0.57 for 6 BZ/2 BLT/92 NKN, respectively. With increasing temperature, the piezoelectric properties decrease. At 200°C, d33, d33( E = 0), S (at 2 kV/mm), and kp are 170 pC/N, 160 pm/V, 0.059%, 0.36 for 5 BZ/3 BLT/92 NKN; and 181 pC/N, 190 pm/V, 0.061%, 0.39 for 6 BZ/2 BLT/92 NKN. It is found that the electromechanical coupling coefficient has a better temperature stability than the piezoelectric coefficient in the studied system due to a large temperature-dependent compliance change. The results demonstrate that engineering an MPB is highly effective in tailoring temperature stability of piezoceramics. [ABSTRACT FROM AUTHOR]

The phase transition temperatures of CaZrO3-doped (Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3 lead-free piezoelectrics ( LKNNT- CZ5) have been studied with X-ray diffraction and Raman scattering. It is found that LKNNT- CZ5 ceramics and powders with the same crystallite size (~2-5 μm) exhibit distinctly different phase transition behaviors versus temperature. The ceramic bulk changes its orthorhombic phase to tetragonal structure abruptly after being pulverized to powders at room temperature( RT), and follows a phase transition sequence with increasing temperature: orthorhombic ( RT-65°C), orthorhombic and tetragonal (65°C-105°C), and tetragonal phase (105°C to ~220°C). Contrastively, the powders hold a tetragonal structure without any phase transition within the whole temperature region ( RT-220°C). These differences might be induced by the existing internal stress or residual strain, which could be released during the grinding process. [ABSTRACT FROM AUTHOR]

The growing environmental concerns have been pushing the development of viable green alternatives for lead-based piezoceramics to be one of the priorities in functional ceramic materials. A polymorphic phase transition has been utilized to enhance piezoelectric properties of lead-free ( K, Na) NbO3-based materials, accepting the drawbacks of high temperature and cycling instabilities. Here, we present that CaZrO3-modified ( K, Na) NbO3 piezoceramics not only possess excellent performance at ambient conditions benefiting from nanodomain engineering, but also exhibit superior stability against temperature fluctuation and electrical fatigue cycling. It was found that the piezoelectric coefficient d33 is temperature independent under 4 kV/mm, which can be attributed to enhanced thermal stability of electric field engineered domain configuration; whereas the electric field induced strain exhibits excellent fatigue resistance up to 107 sesquipolar cycles. These findings render the current material an unprecedented opportunity for actuator applications demanding improved thermal and cycling reliabilities. [ABSTRACT FROM AUTHOR]

The high-temperature phase transition between the tetragonal (scheelite) and monoclinic (fergusonite) forms of yttrium tantalite (YTaO4) has been studied using a combination of first-principles calculations and a Landau free-energy expansion. Calculations of the Gibbs free energies show that the monoclinic phase is stable at room temperature and transforms to the tetragonal phase at 1430 °C, close to the experimental value of 1426 ± 7 °C. Analysis of the phonon modes as a function of temperature indicate that the transformation is driven by softening of transverse acoustic modes with symmetry Eu in the Brillouin zone center rather than the Raman-active Bg mode. Landau free-energy expansions demonstrate that the transition is second order and, based on the fitting to experimental and calculated lattice parameters, it is found that the transition is a proper rather than a pseudoproper type. Together these findings are consistent with the transition being ferroelastic. [ABSTRACT FROM AUTHOR]

The development of lead-free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead-free (K,Na)NbO3 (KNN)-based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficient d33 of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field-induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field-induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature-dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature-insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature. [ABSTRACT FROM AUTHOR]

AgSbO3 was doped into KNN-based lead-free piezoceramics with an optimized composition of Li0.02( Na0.53 K0.48)0.98 Nb0.8 Ta0.2 O3 (abbreviated as LKNNT) to further enhance its piezoelectric property. The doping of AgSbO3 was found to be effective in reducing the grain sizes, resulting in more uniform microstructure in AgSbO3-doped LKNNT ceramics. AgSbO3 lowers tetragonal-orthorhombic phase transition point ( TT-O), but with a more gentle rate as compared with other dopants. A large converse piezoelectric coefficient d33* up to 598 pm/V under a relatively low electric field of 1 kV/mm was obtained in the LKNNT-5 mol% AgSbO3 composition, whose tetragonal-orthorhombic phase transition point ( TT-O) was controlled near room temperature, but its Curie temperature was kept at 235°C. The d33* obtained in the present material is a very high value for nontextured KNN-based ceramics, which is attributed to the polymorphism phase transition effect and 'soft' behavior caused by the addition of AgSbO3. [ABSTRACT FROM AUTHOR]

Abstract: 2% Nb-doped [111]-oriented lead zirconate titanate films (PNZT) of different Zr/Ti ratios ranging from 30/70 to 70/30 with a fixed thickness were deposited on the Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel method. The orientation degree, film morphology and phase transition of the PNZT films were investigated as a function of Zr/Ti ratios. The experimental results verified the composition-dependent phase transition from tetragonal to rhombohedral of the PNZT films with a visible [111] preferred orientation. To obtain high performance by making use of rhombohedrel [111]-oriented thin films, Pb(Zr0.6Ti0.4)Nb0.02O3 films with various thicknesses varying from 70nm to 300nm were studied to reveal the relationship among the film thickness, stress and ferroelectric response. It was found that ferroelectric property reached a high remnant polarization (P r ) of 61.7μC/cm2 and a minimum coercive field (E c ) of 70.7kV/cm at 300nm due to the release of residual tensile stress. [Copyright &y& Elsevier]

( Ba1− x Ca x)( Ti0.92 Sn0.08) O3 ( x = 0.00-0.06 mol) ceramics with a high relative density over 96% were prepared by a conventional sintering method at 1480°C. At room temperature, a polymorphic phase transition from orthorhombic phase to tetragonal phase was confirmed by the XRD patterns in the composition range of 0.04 ≤ x ≤ 0.06. A high piezoelectric coefficient d33 up to 568 pC/N was obtained at x = 0.05 mol, which is higher than the other component of ( Ba, Ca)( Ti, Sn) O3 system. At the same time, the corresponding planar electromechanical coupling factor kp, converse piezoelectric coefficient dS/ dE, and dielectric constant εr reach 47.7%, 1013 pm/V, and 23000, respectively. These results indicate that the ( Ba, Ca)( Ti, Sn) O3 ceramics are a promising candidate to replace for the lead-based piezoelectric materials. [ABSTRACT FROM AUTHOR]

Abstract: A small amount of BiFeO3 (BF) was incorporated into lead-free Li0.03(Na0.53K0.48)0.97Nb0.8Ta0.2O3 (LKNNT) piezoceramics for the purpose of property improvement via phase structure control. It was found that the addition of BF not only induced a series of phase transformations from orthorhombic to tetragonal and further to pseudocubic in LKNNT ceramics, but also enhanced its sintering densification behavior. Even tiny amount of BF addition down to 0.5mol% could change the room-temperature phase structure from a dominant orthorhombic state to the coexistence of orthorhombic and tetragonal phases, resulting in peak piezoelectric properties of , k p ∼47.0%. When the amount of BF was increased to 0.75mol%, the tetragonal phase became dominant in the modified LKNNT ceramics, which exhibited excellent temperature stability, despite its slightly lowered piezoelectric coefficients. [Copyright &y& Elsevier]

Abstract: XRD and Raman scattering experiments revealed an interesting finding that phase structure changed from orthorhombic to tetragonal symmetry when pulverizing (Li,Ta)-doped (K,Na)NbO3 lead-free piezoelectric ceramics (sintered body) to powder. Both orthorhombic and tetragonal phases coexist in the Li0.05(Na0.51K0.49)0.95Nb0.80Ta0.20O3.00 (LKNNT) sintered bulk sample at room temperature, but almost only the tetragonal phase is observed in the ground powder. In addition, annealing experiment enhanced the formation of tetragonal phase and improved the temperature stability of piezoelectricity. It is revealed that the internal stress existing in the LKNNT ceramics favors the formation of orthorhombic phase, which transfers to tetragonal phase when the stress was released. [Copyright &y& Elsevier]

Abstract: Highly dense Li/Ta co-doped (K,Na)NbO3 lead-free ceramics were prepared by spark plasma sintering (SPS) in a relatively wide temperature window. The influences of post-annealing temperature on phase structure, electrical properties and grain growth were investigated. Through tailoring post-annealing temperature, relative dielectric constant ɛ r, and piezoelectric coefficient d 33 of SPSed KNN-based samples can be enhanced to 960 and 225pC/N for the same composition compared with normally sintered values (771 and 206pC/N). The enhanced electrical properties were discussed from the point view of microstructure (grain size and its distribution, porosity) and phase transition, which occurred when post-annealing temperature exceeded 1050°C. Also, with increasing post-annealing temperature, three grain growth behavior stages were investigated on the balance between the volatilization of alkali metal oxides and secondary grain growth. [Copyright &y& Elsevier]

Dense and fine-grained (Na0.535K0.485)1− xLi x(Nb0.8Ta0.2)O3 ( x=0.02–0.07) (abbreviated as NKL xNT) lead-free piezoceramics were fabricated by spark plasma sintering (SPS), followed by postannealing in air. All the NKL xNT ceramics showed single perovskite structures with a phase transition from an orthorhombic symmetry to a tetragonal one across a composition region of 0.04< x<0.05. The sample with a composition of x=0.05 had the maximum values of piezoelectric coefficient ( d33=243 pC/N) and planar electromechanical coupling coefficient ( kp=46.1%), and other good properties such as Qm=85, ℇr=1240, and tg δ=0.023. Because of the enhanced densification and refined microstructure, the SPS-processed NKL xNT ceramics also had better fracture strength than that prepared by normal sintering. [ABSTRACT FROM AUTHOR]

Abstract: To develop high-performance piezoelectric films on conventional Pt(111)/Ti/SiO2/Si(100) substrates, sol–gel-derived highly [100]-textured Nb-doped Pb(ZrxTi1−x)O3 (PNZT) thin films with different Zr/Ti ratios ranging from 20/80 to 80/20 were prepared and characterized. The phase structure, ferroelectric and piezoelectric properties of the PZNT films were investigated as a function of Zr/Ti ratios, and it was confirmed that there was distinct phase transition of the PNZT system from tetragonal to rhombohedral when the Zr/Ti ratio varied across the morphotropic phase boundary (MPB). The Nb-doped PZT films showed enhanced remanent polarization but reduced coercive field, whose best values reached 75 μC/cm2 and 82 kV/cm, respectively at the composition close to MPB. In addition, the [100]-textured PNZT film at MPB also shows a high piezoelectric coefficient up to 161 pm/V. All these properties are superior to those for undoped PZT films. [Copyright &y& Elsevier]

Abstract: A shift of the morphotropic phase boundary (MPB) and a superior piezoelectric response are observed in Nb-doped Pb(Zr x Ti1− x )O3 (PNZT) thin films epitaxially grown on Nb-doped SrTiO3(100) (Nb:STO) substrates. X-ray diffraction and Raman spectra characterizations confirm that a phase transition from a tetragonal structure to a rhombohedral structure occurs when the Zr/Ti ratio varies from 20/80 to 80/20. The phenomenological theory and experimental analyses suggest that the MPB of epitaxial PNZT thin films is shifted to the higher Zr/Ti ratio (around 70/30) from the conventional ratio (52/48) due to the misfit compressive stress induced by the substrate. A maximum local effective longitudinal piezoelectric coefficient (d 33) up to 307pmV−1 is observed at a Zr/Ti ratio of 70/30 in the current compositional range, again confirming the shifting of MPB in epitaxial PNZT thin films. These findings offer a new insight for the fabrication of epitaxial PZT thin films at MPB with a superior piezoelectric response. [Copyright &y& Elsevier]

Abstract: PES–TiO2 composite membranes were prepared via phase inversion by dispersing TiO2 nanopaticles in PES casting solutions. The crystal structure, thermal stability, morphology, hydrophilicity, permeation performance, and mechanical properties of the composite membranes were characterized in detail. XRD, DSC and TGA results showed that the interaction existed between TiO2 nanopaticles and PES and the thermal stability of the composite membrane had been improved by the addition of TiO2 nanopaticles. As shown in the SEM images, the composite membrane had a top surface with high porosity at low loading amount of TiO2, which was caused by the mass transfer acceleration in exposure time due to the addition of TiO2 nanopaticles. At high loading amount of TiO2, the skinlayer became much looser for a significant aggregation of TiO2 nanopaticles, which could be observed in the composite membranes. EDX analysis also revealed that the nanoparticles distributed in membrane more uniformly at low loading amount. Dynamic contact angles indicated that the hydrophilicity of the composite membranes was enhanced by the addition of TiO2 nanopaticles. The permeation properties of the composite membranes were significantly superior to the pure PES membrane and the mean pore size also increased with the addition amount of TiO2 nanopaticles increased. When the TiO2 content was 4%, the flux reached the maximum at 3711Lm−2 h−1, about 29.3% higher than that of the pure PES membrane. Mechanical test also revealed that the mechanical strength of composite membranes enhanced as the addition of TiO2 nanopaticles. [Copyright &y& Elsevier]

A parton and hadron cascade model, PACIAE, is applied to follow the particle transport in both the partonic and hadronic stages of the 0–5% most central Au+Au collisions at $\sqrt{^SNN} = 200\ {\rm GeV}$. We have determined the specific heat of π+ + π− in pure hadronic scenario (hadonic final state) and the specific heat of $u + \bar{u} + d + \bar{d}$ in pure partonic scenario (partonic initial state). The specific heat of π+ + π− in hadronic scenario is nearly a factor of 1.2 smaller than that of $u + \bar{u} + d + \bar{d}$ in partonic scenario. However, the specific heat of $u + \bar{u} + d + \bar{d}$ is very hard to survive the hadronization. [ABSTRACT FROM AUTHOR]

Lead-free alkali niobate piezoelectric ceramics with nominal compositions [(Na0.535K0.480)0.942+xLi0.058](Nb0.90Ta0.10)O3 compensated with Na and K at a fixed and optimized ratio of 0.535:0.480 were prepared by normal sintering. The electrical properties were investigated with a special emphasis on the influence of compensating amount x of Na0.535K0.480. A phase transition from tetragonal to orthorhombic was observed as x was increased from 0 to 0.024, resulting in enhanced piezoelectric coefficient and planar electromechanical coupling coefficient from 216 and 38.1% to 268 pC/N and 46.2%, respectively. Our results indicate the importance of elaborate compositional control for enhancing piezoelectric properties in niobate-based ceramics. [ABSTRACT FROM AUTHOR]

We study a homogeneous two-component dipolar Fermi gas with 1D spin-orbit coupling (SOC) at zero temperature and find that the system undergoes a transition from the paramagnetic phase to the ferromagnetic phase under suitable dipolar interaction constant λ d, SOC constant λ SOC and contact interaction constant λ s. This phase transition can be of either 1st order or 2nd order, depending on the parameters. Near the 2nd-order phase transition, the system is partially magnetized in the ferromagnetic phase. With SOC, the ferromagnetic phase can even exist in the absence of the contact interaction. The increase in dipolar interaction, SOC strength, and contact interaction are all helpful to stabilize the ferromagnetic state. The critical dipolar interaction strength at the phase transition can be reduced by the increase in SOC strength or contact interaction. Phase diagrams of these systems are obtained. [ABSTRACT FROM AUTHOR]

Silver niobate based lead-free antiferroelectric ceramics have demonstrated great advantages, but the high consumption of noble metal silver may restrict their commercial application. In this work, Na+ and Ta5+ co-modified (Ag 1- x Na x)(Nb 1- y Ta y)O 3 (100 x Na-100 y Ta) ceramics were investigated, aiming to reduce the silver consumption and achieve good energy storage properties. The Na+ tended to increase M2-M3 phase transition temperature (T M2-M3), while Ta5+ was more likely to reduce T M2-M3. A new current peak (E R) was observed for the first time in all current-electric field curves. As expected, a room temperature M2-M3 phase boundary with relaxor AFE property was realized in 40Na-65Ta with obviously reduced silver content, in which high recoverable energy storage density (W rec) of 6.5 J/cm3 and good energy storage efficiency (η) of 78% were achieved. This work demonstrates a strategy to realize relaxor antiferroelectrics in AgNbO 3 based ceramics for energy storage performance, and promotes the commercialization potential. • M2-M3 phase boundary with relaxor AFE feature is constructed. • Current peak E R for relaxation is observed in current-electric field curves. • High W rec of 6.5 J/cm3 and η of 78% are achieved in 40Na-65Ta with reduced Ag content. • Promoting commercialization potential of AgNbO 3 -based ceramics. [ABSTRACT FROM AUTHOR]

The phase structure in the (Ba,Ca)(Ti1-xSnx)O3 lead-free ceramics was evolved from inceptive orthorhombic (O) at 0 ≤ x ≤ 0.04 to a two-phase coexistence of pseudocubic-orthorhombic (PC-O) at 0.06 ≤ x ≤ 0.10 and further to a multiphase coexistence of rhombohedral-pseudocubic-orthorhombic (R-PC-O) at x = 0.11. Due to the multiphase coexistence of R-PC-O at room temperature proved by X-ray diffraction, dielectric constant [variant_greek_epsilon]r and differential scanning calorimetry, an ultrahigh piezoelectric coefficient d33 = 670 pC/N and electrostrain 0.061% were achieved. The high d33 over 520 pC/N was stabilized in a wide compositional range of 0.07 ≤ x ≤ 0.11, suggesting that (Ba,Ca)(Ti,Sn)O3 ceramics are a promising candidate for the lead-free piezoelectric ceramics. [ABSTRACT FROM AUTHOR]

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead‐free Li‐modified Na0.5K0.5NbO3 ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic‐tetragonal (M‐T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress‐ and temperature‐dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress‐induced M‐T phase transition in Li‐modified Na0.5K0.5NbO3. A stress‐temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M‐T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure. [ABSTRACT FROM AUTHOR]

0.8(Bi 0.5 ,Na 0.5)TiO 3 -0.2SrTiO 3 (BNT-0.2ST) thin films, with thicknesses ranging from 90 to 364 nm, were fabricated on platinized silicon substrates by sol-gel method. These films were investigated by switching spectroscopy piezoresponse force microscope (SS-PFM) as a function of frequency at room temperature, revealing the enhanced ferroelectric response in ∼ 210 nm film at all frequencies (0.1 Hz - 1.5 Hz). This enhancement was ascribed to the largest thermally-activated stress at such thicknesses generated during film fabrications. As the temperature of the investigated films increases from room temperature to 200 oC, the piezoelectric parameters were obtained from SS-PFM, such as switching polarization (R s), coercive bias (V 0), work of switching (A s), maximum strain (S max), and negative strain (S neg), indicating an occurrence of phase transition from ferroelectrics to relaxors. This work revealed that thickness plays a crucial role for ferroelectric response and temperature-dependent phase transition in BNT-0.2ST films, since it affects the stress state and switching behavior. [ABSTRACT FROM AUTHOR]

Here we observed a large strain of lead-free BiFeO 3 ternary ceramics by the composition design [(0.70- x )Bi 1.05 FeO 3 -0.30BaTiO 3 - x Bi(Mg 2/3 Nb 1/3 )O 3 ]. Positive temperature dependence of strain under E  = 4 kV/mm can be observed in the ceramics with x  = 0.07. Large strain property ( S  = 0.32% and d 33 * = 800 pm/V) and low hysteresis ( H  = 5%) can be achieved in the ceramics measured at 200 °C, which may originate from thermally activated domain switching, electric field-induced relaxor to ferroelectric phase transition, and intrinsic lattice strain. We believe that the lead-free bismuth ferrite ternary ceramics with both large strain properties and small hysteresis are promising for high-temperature actuator applications. [ABSTRACT FROM AUTHOR]

(1- x )(Bi 0.5 Na 0.5 )TiO 3 - x SrTiO 3 (BNT- x ST) (0 ≤ x ≤ 0.4) thin films were fabricated using a sol-gel technique on Pt(111)/Ti/SiO 2 /Si(100) substrates, which were investigated by piezoresponse force microscopy (PFM) and Raman spectroscopy. The composition-induced phase transition was analyzed by acquiring structural variations and the domain distribution on a local scale. Raman spectra showed phonon anomalies with peak broadening and shifting when increasing SrTiO 3 (ST) concentrations were used. Changes in the domain morphology with changes in the composition were observed, and grains smaller than 0.5 µm were observed at lower concentrations of x = 0–0.25, while larger grains appeared with increasing ST contents. The switching spectroscopy PFM (SS-PFM) results supported a ferroelectric (FE) to relaxor ferroelectric (RFE) phase transition at approximately x ≈ 0.3 by means of analyzing the parameters as a function of the composition including the piezoresponse parameters of hysteresis loops ( D max , D rem ) and amplitude butterfly loops ( S total , S neg ). Hence, these results demonstrated that the composition-driven FE to RFE phase transition behavior, which is consistent with the localized response behavior, is dependent on the ST content in bulk BNT- x ST ceramics. [ABSTRACT FROM AUTHOR]

0.25wt% CuO-doped (Li,K,Na)(Nb,Ta)O3–AgSbO3 lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000°C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100°C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal–orthorhombic phase transition point (T T−O ) but a lower Curie point (T c ), compared to undoped ones. The KNN-based piezoceramics became “hard” as CuO was added, supported by an increase of Q m . Fairly good electrical properties of d 33 *=383pm/V, ε r =860, Q m =188 and T c =215°C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970°C, demonstrating promising potential in practical applications. [ABSTRACT FROM AUTHOR]

We have investigated the structural phase transitions of crystalline Si (insulator-metal), SiO2 (insulator- insulator), and Zr (metal-metal) under pressure, as a test of several density functionals for the exchange- correlation energy. While meta-generalized gradient approximations (meta-GGAs) such as revTPSS (revised Tao-Perdew-Staroverov-Scuseria) are more sophisticated than GGAs such as PBE (Perdew-Burke-Ernzerhof), and are more accurate without empiricism for atomization energies of molecules, lattice constants of solids, and surface energies, we confirm that these meta-GGAs tend to give smaller and less realistic transition pressures than the PBE GGA does. But we also show that the recent functionals of the meta-GGA made simple family (MGGA_MS) behave differently, predicting larger and often more realistic transition pressures. We suggest that further refinement of the meta-GGA can lead to a functional that is more accurate for properties of molecules and solids at equilibrium or under compression. We also show that, contrary to recent suggestions but in line with older ones, an accurate fundamental gap in the noninteracting band structure is not necessary for an accurate prediction of the transition pressure. Unlike the semi local GGAs and meta-GGAs, and unlike the local density approximation also tested here, the screened hybrid functional HSEO6 (Heyd-Scuseria-Ernzerhof) is fully nonlocal and predicts more realistic fundamental gaps. HSEO6 is better than the semilocal functionals for the transition pressures of Si and SiO2, but seriously overestimates the transition pressure in Zr. Besides the transition pressures, we report the transition energies and volumes, binding energy curves, and structural parameters at zero and transition pressure. Finally, we discuss how the performance of a functional can reflect its plottable exchange enhancement factor, and why the structural phase transitions are especially challenging for approximate density functionals. [ABSTRACT FROM AUTHOR]

We have assessed the performance of the non-self-consistent random phase approximation (RPA) on two pressure-induced structural phase transitions, diamond to β-Sn Si in Si and α-quartz to stishovite in SiO2 The calculated equilibrium lattice properties of the four structures are in better agreement with experimental results than are those from several semilocal functionals. The energy differences between the high- and low-pressure phases are found to be 0.37 eV/Si and 0.39 eV/Si02, respectively. The transition pressure obtained from our RPA calculations for diamond to β-Sn in Si is 12.2 GPa, in excellent agreement with the experimental value 11.3-12.6 GPa. However, the α-quartz to stishovite phase-transition pressure in S1O2 is found to be 5.6 GPa, lower than the experimental 7.46 GPa; the Perdew-Burke-Ernzerhof (PBE) semilocal functional gives the transition pressure closest to experiment in this case. We conclude that the non-self-consistent, nonlocal RPA accurately describes the insulator-to-metal transition in Si, where semilocal density functionals tend to fail. But the RPA error cancellation that is nearly perfect in many solids, including Si, may be less perfect in solid SiOi, as it is in many molecules. [ABSTRACT FROM AUTHOR]

Abstract: ZnO-doped Li0.06(Na0.535K0.48)0.94NbO3 ceramics were synthesized by a conventional mixed-oxide method. A dense microstructure was achieved and was free from deliquescence when ZnO was added. With adding 0.6 mol% ZnO, the Zn2+ first enters the A-site, while higher amounts of Zn2+ substitutes B-site ions. The addition of ZnO changes the ceramics to ‘‘hard’’ performance with improvement of Qm to 235. A phase transition from tetragonal to orthorhombic phase was obtained in the compositional range of 0.6–0.8 mol% ZnO. Because of such transitional behavior, d 33, kp , and εr reached peak values of 250 pC/N, 45%, and 986, respectively. [Copyright &y& Elsevier]